One should always be careful of generalizations. Its all about the seals!
While the 3-500Z indeed has a reputation for being a "leaker," you also find
anecdotes about tubes that have been on the shelf since the 40's coming up
just fine. Caveat: its always a good idea to bring them up slowly!
Indeed, small receiving types very rarely leak. Of course they generally
don't have kilovolts of plate voltage, which might contribute to this
reputation.
A couple of anecdotes from my life as a broadcast engineer.
When I started working at KRLA (AM)in the 70's, they had the two oldest
transmitters in the market. One of them was a truly amazing 10kw Western
Electric. It had a pair of big water-cooled glass triodes for the final (it
was a Doherty configuration). We had these tubes rebuilt over and over again
by Cal Tube Labs and they were fine. Then all of a sudden, they wouldn't
hold vacuum and would arc after being shut down for a mere 12 hours or so
(the transmitter was a backup that was run in the over-night period or if
the main failed). To make a long story short, it turns out that the fellow
that had been rebuilding these at Cal Tube Labs died, and apparently took
some knowledge with him. That spelled the end of life for that transmitter.
The other transmitter was a 1950's Continental 317. The driver section had a
pair of Machlett 5531 external anode glass triodes. By the time I arrived,
these tubes only had about 1500 good hours. They also would arc once at
initial turn on. According to the supervisor (Tom Weatherall - SK), the
problem with the tubes started when, as part of an anti-trust settlement,
Western Electric had to divest all its broadcast interests. Their tube
division was Machlett labs, which was sold to Raytheon. They couldn't build
a clean 5531 tube after that change. Unfortunately, the elaborate glass
design meant nobody wanted to attempt to open the tubes for a rebuild.
While ceramic tubes appear to held vacuum better, it probably has more to do
with the manufacturing knowledge and skill set for the seals.
Chris, AB6QK
ORIGINAL MESSAGE:
Message: 5
Date: Tue, 6 Jun 2017 15:14:26 +0000 (UTC)
From: Catherine James <catherine.james@att.net>
To: Amps group <amps@contesting.com>, Kimberly Elmore
<cw_de_n5op@sbcglobal.net>
Subject: Re: [Amps] 3-500Z cool down time
Message-ID: <1744412737.2823165.1496762066255@mail.yahoo.com>
Content-Type: text/plain; charset=UTF-8
Kim,
It's generally recognized that glass tubes have poor shelf life due to
slow leakage. I have heard many reports of this from people I trust.
They will last much longer if the plate gets hot at least a few times a
year so that any leakage can be gettered away. For tubes in regular use
rather than on the shelf, it doesn't seem to be a problem, but it makes it
difficult to stock up with spares. Many amateur users rotate their shelf
stock into the amp at least once a year.
Is the 3-500Z used much outside of ham radio today? If so, who are the
primary users?
73,
Cathy
N5WVR
--------------------------------------------
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|